The invention relates to rotary drill bits for use in drilling or coring deep holes in subsurface formations and, in particular, to arrangements for mounting cutting members in such bits.
Rotary drill bits of the kind to which the invention relates comprise a bit body having a shank for connection to a drill string and an inner passage for supplying drilling fluid to the face of the bit. The bit body carries a plurality of cutting elements. Each cutting element may comprise a circular preform having a thin hard facing layer, which defines the front cutting face of the element, bonded to a less hard backing layer. For example, the hard facing layer may be formed of polycrystalline diamond or other superhard material, and the backing layer may be formed of cemented tungsten carbide. The two-layer arrangement of the cutting elements provides a degree of self-sharpening since, in use, the less hard backing layer wears away more easily than the harder cutting layer. However, single layer preforms are also known and have the advantage that they may be thermally stable.
In the type of drill bit to which the invention relates, the cutting elements are mounted on the bit body by being bonded, for example by brazing, to a carrier which may be in the form of a stud of tungsten carbide which is received and located in a socket in the bit body which may be formed, for example, from steel or from a tungsten carbide matrix.
Conventionally, the studs on which the cutting elements are mounted are secured within their respective sockets by brazing, press fitting or shrink fitting. While press fitting and shrink fitting are suitable for steel bit bodies where the sockets may be fairly accurately machined, difficulties arise in using such methods with a matrix body. In view of the difficulties in machining tungsten carbide matrix, the sockets are usually molded in the surface of the bit body at the same time as the bit body is formed. However, this means that the dimensions of the sockets cannot be accurately controlled according to the tolerances necessary for press fitting or shrink fitting, with the result that studs may be inadequately secured within the sockets or attempts to hammer the studs into an undersize socket may lead to cracking of the bit body.
Attempts have been made to overcome this problem by moulding the side walls of the sockets in a manner to give a textured surface so as to increase the permitted tolerances to give a satisfactory interference fit, but such methods have not proved entirely satisfactory. The problem has normally, therefore, been overcome as far as matrix bits are concerned by brazing the studs in the sockets, but it will be appreciated that this adds to the cost of manufacture of the bit.
The present invention sets out to provide an improved form of mounting for the carriers of preform cutting elements in a bit body.